1. Field of the Invention
The present invention relates to a power supply and a method for compensating low-frequency output voltage ripple and, more particularly, to a technique pertinent to control command compensation performed by providing zero-crossing information of AC (Alternating Current) power of a power factor correction (PFC) circuit on a primary side of a power supply and load information to a DC (Direct Current) to DC conversion circuit for the DC to DC conversion circuit to compensate control command thereof in collaboration with a table-mapping means.
2. Description of the Related Art
With reference to FIG. 6, a conventional switching power supply has a rectification circuit 81, a PFC circuit 82 and a DC to DC conversion circuit 83. The rectification circuit 81 converts an AC input power (AC in) into a DC power. The PFC circuit 82 is connected to an output terminal of the rectification circuit 81, and has an inductor L1, a diode, a first power switch S1 and a PFC controller located on a DC circuit loop. The PFC controller is connected to and detects the AC input power (AC in). The DC to DC conversion circuit 83 has a transformer T1, a DC to DC controller U1 and a second power switch S2. One control terminal of the DC to DC controller U1 is connected to the second power switch S2. The second power switch S2 is connected to the primary side of the transformer T1.
The DC to DC conversion circuit 83 acquires an input voltage Vin(t) from an output terminal of the PFC circuit 82, and further generates an output voltage Vout(t) after conversion. The input voltage Vin(t) is converted from the AC input power (AC in) at a specific frequency (e.g. 60 Hz) and thus contains low-frequency ripple at a frequency doubling the specific frequency (e.g. 120 Hz) as shown in FIG. 7A. The low-frequency ripple still exists in the output voltage Vout(t) as shown in FIG. 7B even after the conversion of the DC to DC conversion circuit 83. However, the low-frequency ripple contained in the output voltage Vout(t) should be removed or reduced as much as possible.
A first approach of eliminating low-frequency ripple in the output voltage Vout(t) is to increase the low-frequency response speed of the DC to DC conversion circuit 83. As such approach involves higher complexity of the DC to DC controller U1 and measurement of input voltage and may affect other low-frequency response characteristics, directly increasing the low-frequency response speed of the DC to DC conversion circuit 83 is not an ideal solution.
With reference to FIG. 8, a second approach targets at adding a ripple suppression circuit 84 between a DC input voltage terminal and the DC to DC controller U1. The ripple suppression circuit 84 has a high-pass filter 841 and an adder 842. After passing the high-pass filtering, the input voltage Vin(t) is added to a reference signal by the adder 842 to compensate control commands of the DC to DC controller U1 and thereby eliminating the low-frequency ripple contained in the output voltage Vout(t).
Although the second approach can eliminate low-frequency ripple in the output voltage Vout(t), the input voltage Vin(t) is a high-voltage DC power and the ripple suppression circuit 84 also involves a high-voltage circuit loop, which makes the circuit design more complicated and inevitably increases the production cost. A third approach, which is similar to the second approach, adopts a resonant controller to replace the foregoing ripple suppression circuit 84. However, the third approach also has the issues of higher circuit complexity and cost.